Friday, July 17, 2015

Temperature Compression

I build my caster product using a piece of aluminum tube with a piece of acetal that carries the caster and slides inside the tube.  Right now, because this is a 30 degree angle cut, I cut the acetal, then sand the edges of the nominally 1.75" plastic (actually more like 1.755") to fit into the nominally 1.75" ID tubing (actually more like 1.741" ID).  Once inside, I screw through the aluminum into the plastic to hold it there.  I am getting ready to switch to aluminum for the insert that holds the caster.  I can turn a rod of aluminum  to the required diameter on the lathe, then cut the rod into the 30 degree angle slices.

I am tempted to use heating and cooling expansion to get a more precise fit.  This calculator shows thermal expansion for various materials.  It looks like putting the interior part on dry ice (-78 C.) should shrink 1.741" OD to 1.7371".  Similarly, putting the 1.741" ID tubing in the oven to 200 degrees C. will expand it to 1.7482".  The cold piece then slides easily into the tube.  Then dunk everything into cold water, and the dry ice piece and hot piece expand and contract to a very tight fit at 1.741".  Question:  how strong is this?  Do I still need screws?   Eliminating drilling, tapping, and three 8-32 screws might justify the nuisance of using oven mitts for the hot from the oven and dry ice.

I put a 6" long piece of 1.751" OD rod on the dry ice and it started moaning!  Now 1.739" OD. 
Last measure: 1.745" OD, perhaps because frost is accumulating on it, or chunks of frozen skin.  (Oven mitts do wonders!)  The tube this goes into is 1.72" ID at near room temperature.  I am confident that oven temperatures will expand it enough to take the rod, and heat transfer should do the rest. I may drill, tap, and screw anyway just to be sure, but at least there is no danger of the rod slipping out of the tube.

UPDATE: I baked a PIECE OF 1.7505" id tube at 450 deg. F., and  it grew to 1.754" ID.  Now, I have trimmed the piece of rod to 1.746" ID at dry ice temperatures.  This should slip into the oven-baked goodness tube, and then both ram into each other at 1.7505".  I still need to slice yhe rod inton 30 degree slices, and try this out.

7 comments:

Jim Dunmyer said...

A shrink fit is VERY strong, and would probably eliminate any need for screws. You will want a .001"+ interference fit. The rule of thumb for steel is .001" per inch of diameter, but I don't know what aluminum calls for. Machinery's Handbook will tell you, or a quick Internet search should help. In this particular case, with relatively thin-walled tubing on the outside, you could err on the side of MORE interference, IE: .002".

I've never personally bothered with using a freezer or dry ice, just heat. Hint: set your telescoping gauge for the desired I.D. size of the outside part and check it (while hot) before sliding things together.

Another alternative is anerobic adhesive (LocTite), and a slight clearance fit. I've used SleeveLocker on such stuff with very good results. Note that LocTite has an expiration date that you should observe.

bud said...

I can't tell you the numbers, but it is a worthwhile investigation; the wheel bearing on my mc are assembled using this method: hub with the heat gun and the bearings in the freezer. Tap them into place, and when they are the same temperature, it takes a lot of hammering to get them out.

Will said...

Partly that would depend on the length of the overlap. Might I suggest you look at Loctite's products. There should be an appropriate adhesive that would permanently hold the parts together. Consider talking to them with your specifics, to get an optimum choice.

One question that should be looked at is how uniform are those dimensions held, both on an individual piece, and from run to run, from the manufacturer? It can really screw things up when you design a part that uses a vendor's stock part dimensions, only to have them change it, or cease making it. Some of those adhesives can deal with large gaps, and some can't. Do a bit of research on this before making any decisions.

Needless to say, but I will anyway, you should spot check every new batch of material for dimensional changes, since that could be a big problem, no matter which method you end up using.

Bob said...

A pretty straight forward analysis can be found at http://www.faculty.fairfield.edu/wdornfeld/ME311/PressCylinderHam.pdf

At the end there are formulas for maximum axial and torque loads that can be carried. It appears to be amenable to a spreadsheet implementation. "Properly" sized pieces fitted this way will *not* require additional screws to hold it together. Highly recommend doing the analysis which should include min/max numbers for the radii of the inner and outer pieces at "room" temperature (leading to min/max stresses and load carrying capability). Pay attention to the stresses: it is possible to achieve stresses that will cause the outer piece to fail.

Jim Horn said...

Aluminum always has an aluminum oxide coating which prevents the rest of it from oxidizing. The bad news is that the structural integrity of that can vary tremendously. One result is that bonding things to aluminum has long been known to be problematic especially if long term reliability is required (such as bonding aluminum aircraft parts together!).

In aviation, specific oxide forming procedures are used (phosphoric acid anodizing - PAA - is specified for military use).

I may be a worry-wart, but since few adhesives will withstand hot/cold joining as you describe, and I'm sure you don't want customers to suddenly find these joints coming apart. So I'd go with the heat approach or deal with a vendor with professional levels of advice to ensure that the adhesive used is really made for the alloys you are using.

No, I'm not a machinist or mechanical engineer but worked closely with a friend who spent years looking into this before building a bonded-structure aluminum homebuilt airplane. The design looked sound but some examples literally started falling out of the sky. He had his parts PAA treated by one of the defense grade firms and his didn't have such problems, thank God.

Best to you and yours, Clayton!

Clayton Cramer said...

Jim not planning to use adhesive. If temperature compression is not enough then screws.

Will said...

I would be leery of using that high a temp on aluminum. Depending on the specific alloy, it could turn plastic and take a permanent set. IIRC, you shouldn't have to go that high in temps to reach a nominal maximum growth in dimensions.

You might consider doing that angle cut after you join the two pieces, so you don't have to worry about indexing the two parts.